Rectangle plates crack in half.

[d.j. kerschner]

I have been making rectangular trays for a few years. Lately they have been cracking in half on the 3rd firing.  I bisque at cone 05, then fire at cone 6 (Coyote glass mostly) and refire with decals to cone 05. I roll the slabs, carefully, then use the foam and press method to shape the trays. The trays are uniform thickness, not too thin or thick. I dry very slowly, between 2 pieces of backer board and with plastic for several days, then just thin plastic until dry and a few more days to totally dry. I put them on small "chicklets" of fired clay in the kiln, allowing air under the piece and also giving them a chance to shrink. I have been using several different ramps, all with a preheat and a slow cool. I am using Standard 240 white stoneware, but have had no problems with cracks in any other pieces. I have also researched and can't find any glaring errors in my methods. I am, frankly, stumped at this point.

[Min]

What’s your decal firing schedule? Could be too fast for vitrified clay.

welcome to the forum!

[Mark C.]

Once they are tight(fired to cone 6) they need a slow up down cycle for the decal fire-Min hit on the head above

remember if it was easy everyone would  be doing it.

[neilestrick]

Refires gotta go slow, especially for flat pieces as they don't heat very evenly. Since your clay shouldn't deform in the 05 decal firing, you may want to put them up on stilts so they heat and cool more evenly. You could use pieces of kiln shelf or little clay wads as stilts.

[Sorcery]

I don't trust drying between 2 things. I understand it can work and is prescribed often, but say your boards and slab do this (l) only horizontal, you pinch each end, and when it shrinks during drying, it gets weak somewhere.

Sorce

[Bill Kielb]

2 hours ago, Sorcery said:

don't trust drying between 2 things. I u

Definitely why I always put a cheap cfold recycled paper towel under things. Super economical way to make sure it slides nicely and drys evenly. I no longer will set something on a ware board without a cfold towel underneath.

[Babs]

Drying vetween boards excellent for tiles.

But not so for any forms with depth. 

[Jen WC]

On 12/5/2020 at 1:50 AM, Bill Kielb said:

Definitely why I always put a cheap cfold recycled paper towel under things. Super economical way to make sure it slides nicely and drys evenly. I no longer will set something on a ware board without a cfold towel underneath.

Bill,
What is a "cfold" please? Otherwise - sounds like great advice! 

[Callie Beller Diesel]

@Jen WC They’re those brown folded paper towels you usually find in public washroom dispensers. Sometimes called tri-fold.

[Bill Kielb]

2 hours ago, Jen WC said:

Bill,
What is a "cfold" please? Otherwise - sounds like great advice! 

 

1 hour ago, Callie Beller Diesel said:

@Jen WC They’re those brown folded paper towels you usually find in public washroom dispensers. Sometimes called tri-fold.

Yep, cheap, recycled and drys out the bottom about as quick as the sides while letting the pot slide easily..  Moisture wicks away. Lots of times I just leave it on the ware board often good for several wares over time. Most economical thing I have ever used.